In athletic and leisure-time wear, soles made of natural rubber, closed cellular or foam structures and like cellular materials are widely used because their elasticity or resiliency imparts a springiness to the step of the user that is particularly desirable in such applications.
While efforts have been made in the past to include air cells in such soles, such efforts have proved to be unsatisfactory because, on the one hand, it is desirable to be able to adjust the pressure of air in the air cell to vary the resilience of elasticity for the particular purpose or use. Furthermore, stability of the insole has proved to be a problem where a cell was provided in the interior of the insole and, in general, insoles with high resilience have tended to be associated with shape instability.